Batteries are used to power various types of systems. For instance, one or more batteries may be used to power electrical systems such as those used in or associated with power electric vehicles (EVs) or hybrid electric vehicles (HEVs). The batteries used in these systems can be configured as packs of batteries that have voltages in the 150-400 volt range.
However, the accessories that are used in vehicles typically require voltages much less than 150-400 volts. As such, the vehicle must have a conversion apparatus that reduces the battery voltage from the 150-400 volts to a smaller value. One example of such a conversion device is a DC-DC converter. In some examples, the DC-DC converter reduces the voltage from the 150-400 volt range to a 12 volt value range (10-16V)
One example of a DC-to-DC converter is an LLC resonant converter. Typically, the LLC resonant converter utilizes two inductors and a capacitor that couple to a transformer. One of the inductors (Lm) can be integrated into the transformer as the magnetizing inductance The LLC resonant converter transforms an input voltage (e.g., 200 volts) to a voltage value that can be used by accessories of the vehicle (e.g., 12 volts).
Although DC-DC converters are used to transform voltages, they have several operational limitations and issues. For instance, battery voltages may vary over time and it is still desired to keep up the same output voltage so that the accessories can function. The output voltage needs to be controlled depending on temperature and accessory operation. The devices also need to be low cost and efficient, or the vehicle will become prohibitively expensive to purchase and/or operate.
Previous approaches at overcoming these limitations have generally had various drawbacks. Typical operation of an LLC converter is to vary the switching frequency in response to load and/or input voltage variation. Using frequency alone limits the range of input voltage from minimum to maximum. As load current is reduced on the 12 V, the output voltage tends to rise. To compensate, the frequency has to be increased to correct the voltage. In this type of converter the frequency would have to increase multiples of the nominal switching frequency. This increased frequency causes increased switching losses of the switching FETs and output diodes and loss of efficiency.
Another prior approach for the light load condition is to skip a cycle. That is, to apply a maximum frequency that has minimum switching loss and to turn on and off that frequency at a slower rate than switching frequency to regulate the output voltage. As the load is reduced, the skip cycling causes ripple on the output voltage as well as possible audible noise from the converter. The skip cycling method still has a limited input voltage range.
Another prior approach is to apply a minimum load resistor in the converter so the load is never reduced enough to create these undesirable conditions. This extra load resistor reduces efficiency of the converter and is not desirable. Most of these designed LLC converters are limited in input voltage range as well as load range.
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